The Framingham Heart Study began in 1948 and followed over 5,000 participants for decades. The volunteers made up 40% of the population of Framingham, Massachusetts.

In 2003, Nicholas Christakis, a social scientist and internist at Harvard, and James Fowler, a political scientist at UC San Diego, began searching through the Framingham data. But they didn't care about LDL cholesterol or enlarged left ventricles. Rather, they were drawn to a clerical quirk: The original Framingham researchers noted each participant's close friends, colleagues, and family members.

"They asked for follow-up purposes," Christakis says. "If someone moved away, the researchers would call their friends and try to track them down."

Christakis and Fowler used the social data to study changes in the population over time. They constructed networks of the volunteers social connections to see how these connections affected any changes. The findings? Some behaviors are contagious. Social connections with up to three degrees of separation influence whether we quit smoking or become fat. And even happiness is contagious, both online and offline. The social connections of the Framingham volunteers are graphically illustrated at Wired. Link